It is more convenient to be able to power portable devices without the need to plug in a traditional power cable into the device. For example some wireless power systems include a portable device that when placed near a wireless power supply unit can receive power without the need for a direct electrical contact. However, when there is no device on the unit (or when the only devices on the unit are fully charged) it is possible to keep the power consumption at a minimum.
Some wireless power units have a standby mode, whereby it periodically transmits power for a short period to look for devices. If the unit detects a valid device that is requesting power and determines that there are no foreign objects that would get hot or hinder power transfer in the vicinity then the unit may come out of standby. The power level of the pulses in standby mode are sufficiently high to transfer enough power to the portable device so that it can communicate back, because it is possible that the device's batteries may be fully depleted. The length of the pulses are long enough to determine that the device is a valid device and that there are no foreign objects present that may get hot or otherwise interfere with the system. The time between the pulses is short enough that the user gets quick feedback that the unit is operational. There is therefore a limit to how low the power consumption can be during standby.
In addition to the power for determining the presence of devices to be powered, there are practical limitations that increase the power consumption. For instance, in some inductive power supplies a DC power source is used even between the transmitted pulses. This means that mains rectification losses are always present and can be considerable. In some scenarios, multiple DC voltages are used and it would not be practical to start these up within the pulse duration, so DC conversion losses might always be present. The microprocessor in the unit that drives the pulse width modulation for the inverter typically is relatively high performance and consumes a certain amount of power continuously.
These and other factors make it challenging for a wireless power system to have low standby power. One attempted solution is to have a switch, so that the user switches the unit on before placing a device on it. However, this considerably detracts from the main benefit the system provides—the convenience of just putting the device on the wireless power supply. With careful design it is possible to achieve standby powers as low as 0.5 W. However, there is a desire for these figures to be reduced further. A typical mobile phone charger may only be used for 3 hours a week and spend the rest of the time in standby. Assuming an average of 4 W consumption during charging and 0.5 W during standby, the annual energy consumption would be 0.624 kWH to charge the phone and 4.38 kWH whilst in standby. This means that seven times as much energy is wasted in standby compared to the energy used. The impact of shipping 100M units (10% of annual cell phone sales in 2007) would mean approximately 50 MW of power generation capacity just to service the standby. There is increasing awareness that energy wastage through electronic devices left on standby may contribute to climate change. As a result there are initiatives to reduce the power consumption of electronic devices whilst in standby.
There have been several devices aimed at reducing the standby power of televisions and other appliances using remote controls (U.S. Pat. No. 6,330,175, WO2006106310). However, these are not applicable to wireless power systems.
In addition to wireless power systems, other systems for example contactless card systems also suffer from power wastage in standby.